Recently, a virtual technique in which a plurality of virtual machines are installed on a physical server and that makes each of the virtual machines run an arbitrary operating system (OS) and arbitrary applications has come into practical use. In a virtual machine environment, for example, when a physical server is maintained, a virtual machine running on the physical server can be migrated to another server and provide a service from the physical server to which the virtual server has been migrated (for example, see Japanese Unexamined Patent Application Publication No. 2009-26117).
Here, an operator who migrated the virtual machine recognizes to which server and from which server the virtual machine has been migrated. However, other operators cannot determine whether the virtual machine on the physical server, from which the virtual machine was migrated, disappears as a result of migration or deletion of the virtual machine unless they investigate specifically the operation statuses of virtual machines on physical servers.